Hospital bedrail counterbalance unit



Aug. 20, 1963l F. D. WRIGHT HOSPITAL BEDRAIL COUNTERBALANCE UNIT Filed Nov. 17. 1961` III,l

(/"Miiiiuiii INVENTOR.

lPEDEFQICK D. WRIGIHT ATTORNEY `articles and vother accessories.

3,100,899 HOSPITAL BED @COUNTERBALANCE UNIT Frederick D. Wright, 1229 Maddox St., Long Beach 10, Calif. Filed Nov. 17, 1961, Ser. No. 152,9538 8 Claims. (Cl. 5-92) 'Ihe present invention relates generally to the iield of hospital accessories, :and more particularly to an improved bedrail counterblalance unit that permits a bedrail to be raised and lowered with a minimum iotE physical exertion.

A relatively recent development in hospital beds is to provide a cabinet that is mounted on one of the bedrails, with this cabinet being adapted to house such items as a radio, telephone and :storage space for a patients toilet Such cabinets may weigh trom thirty to forty pounds, and inasmuch as they are mounted on the bedrails, they move upwardly yand downwardly with the rails Ias they are pivoted from a protective position to a non-protective position on the hospital bed. Due to .the combined weight 'of the cabinet and the bedrail structure, it is a not uncommon occurrence for the bedrail supporting the cabinet to slip from a nurses hands during manipulation :ot the bedrail whereby the cabinet lfalls to the floor, causing severe damage thereto. Although the cabinet as above described is a distinct convenience tor the patient, the combi-ned weight of the cabinet and bedrafil works ra hardship on the attending nurses in moving the cabinet-supporting bedrail lfrom a downwardly non-protective position to an upwardly protective position where the bedrail protects the patient and also places the cabinet in an operative position where it may be utilized by the patient.

A major vobject of 'the present invention is to provide a mechanism that may be removably mounted yon a hospital b ed equipped with a bedr-ail that supports a cabinet of the character described, with the spring action of this mechanism compensating dor the combined weight of the bedrail and cabinet mounted thereon to permit the same to be pivoted either from a protective to a non-protective position `or vice versa, with a minimum of physical exertion.

Another object of the invention is to supply a device that can be used with a hospital bedrail, irrespective of whether a cabinet is supported therefrom, whereby lthe physical eliort required to manipulate the bedrails is reduced to a minimum.

A still further object of the invention is to provide an extremely simple mechanism which is easily mounted on a conventional 'hospital bed to compensate lfor the weight of the bedr-ail irrespective of the positioning thereof, requires a minimum of maintenance attention, and can be fabricated from standard, commercially available materials whereby it can be sold at a suiciently low price as to encourage its widespread use in hospitals, sanitariurns and like institutions.

A further object of the invention is to provide a device that counterbalances ya bedrail with a cabinet mounted thereon to the extent that nurses `ido not violate the state health fand safety laws relative to lifting when they pivot the Ibedrail to fan up or down position.

'Ihese `andother objects and advantages of the invention will become apparent `from the :tol-lowing description of a preferred form thereof, and Ifrom the accompanying drawing illustrating the same, in which:

FIGURE l is -a perspective View of la conventional hospital bed equipped with bedrails, on 4one of which la cabinet is supported, with the weight :of both of the bedrails as they are pivoted between lan up and down position being compensated :for by the counterbalance lunit of the present invention mounted 'on `the headboard of the bed; t

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FIGURE 2 is yan exploded perspective view of the counterbalance unit :shown in FIGURE 1, from which the cover has been removed;

FIGURE 3 is a iront elevational view of the counterbalance unit shown in FIGURE v2, trom which the cover has been removed;

FIGURE 4 is a top plan view of the unit illustrated in FIGURE l with portions thereof being broken away to show the interior of the invention;

FIGURE 5 is an enlarged side elevational View of that portion of the unit shown on the rig'hthand side of FIG- URE 3;

FIGURE 6 is a fragmentary vertical cross-sectional view od the countenbalance unit taken on line 6-6 of FIGURE 1;

FIGURE 7 is a vertical cross-sectional view of Ithe unit taken on line 7 7 of FIGURE 9; p

FIGURE 8 is a horizontal cross-sectional view of a portion lof the unit taken on line -8--8 of FIGURE 3;

'FIGURE 9 is a perspective view of an end portion of a hospital bed on which one of the bedrail counterbalance units is mounted;

FIGURE 10a is a perspective view of a iirst component used with an anchor forming a part of the unit; and

FIGURE 10b is fa perspective view of a second component of the anchor.

With continuing reference to the ydrawing for the general arrangement of the invention, it will be seen that the counter-balance unit, generally designated by the letter A is mounted on the headboard B of a conventional hospital bed C, best seen in FIGURE l. The Ibed C includes a tootboard B that is of the same :general structure as headboard B. Headboard B comprises two vertical corner posts E, ia lower cross piece F that rigidlyconnects fthe posts, and ra longitudinally extending member G which connects the cross piece lF 'and similarly situated cross piece (not shown) form-ing a part of the ootboard B.

A cross member H connects the upper portions :orf the posts E, as best seen in FIGURE l. A panel I of sub'- stantial 'depth extends transversely between the post E and is rigidly aixed thereto. Panel I (FIGURE l) is set inwardly a substantial distance rirom the forwandmost surfaces of tlhe posts E. 'Ilhe posts E have downwardly extending members K which are adjustably supported therein by means (not shown), which members terminate in bifurca-ted arms 10 between which la shaft 12 extends and The bed C may best be seen in FIGURE l,` and isprovided with two bedrails, with the bedrail on thel left-hand side of the bed being identied by the letter M and the right-hand rail by the letter M. The bedrails M and M are identical in construction, with the exception that at substantially the upper center of bedrail M a cabinet N is supported that contains a radio, telephone, storage space for toilet articles and the like, that may be needed `by a patient occupying the bed on which the bedrail M ismounted. Bedrail M includes a longitudinally extending rigid tu'bularmember 15 that curves downwardly on the ends thereof to dene two legs 17 having inwardly projecting extensions 18 which are pivotally supported by pins 16. The pins 16 are mounted on brackets 19 that are aixed to the cross pieces F, as may best be seen in be explained hereinafter. l comprises a cylindrical body 30 having a threaded pro- FIGURES 1 and 9. Tubular member 15 has two longitudinally spaced arms a extending upwardly therefrom which have longitudinally positioned bores formed in the upper ends thereof that are engaged by pins 28.

Thepins 211 falso engage the lower ends of two upright-s 22 that have upper and lower rigid members 24 and 26 extending therebetween as may be seen in FIGURE 1.

toward the headboard B, `also shown in FIGURE 1. A longitudinally extending tapped recess 27 is formed in extension 24a, as may best be seen in FIGURE 6.

A connector 28 is provided, the function' of which will Connector 28 (FIGURE 6) jection 32 that engages the tapped recess 27. An elongate member 34 having a bore 36 `formed therein that extends transversely therethrough is gripped between the adjoining ends of extension 24a and body 30 when the threaded projection 32 engages tapped recess 27 `as shown.

, pins `4i?, the bedrails MV and M may be pivoted downwardly to occupy the position shown in phantom line in FIGURE 1. The body 30 has ra longitudinally extending cavity 42 formed 4therein 'as well as a tapped transverse bore 44 that is in communication with this cavity. A screw 46 is mounted in the tapped cavity 42.

The fcounterbalance unit, as may best be seen in FIG- URE 3, includes two arms P and P' that extend upwardly and outwardly in opposite directions therefrom. Slots 48 and 48 `are formed in the outer extremities of arms P and P. The slot 4S can be engaged by a fastener 50 that includes a head 52 from which a shank 54'cxten'ds. Shank 54 passes through the slot 48 and thence through a washer 56 to snugly engage the cavity 42. Shank'54 khasa flat area 5S formed on one side, which `flat area may be pressure-contacted by the interior end of the screw 46 to holdthe fastener in a fixed position within thereonfines of cavity 42. The same structure just described is also utilized inI connecting the arms P to the endportion of the bedrail M most adjacent the headboard B.

The counterbal'anee unit `also includes a housing 60, best seen in FIGURES 2 and 3, that is defined by a transversely extending web 62 to an upper flange 64 and lower ilange 66 that projects therefrom. Flange 66 has a rectangular recess 68 formed in the outer part of each end portion thereof. Flange 66 also has an extension 70 that projects upwardly from the -outer longitudinal edge thereof between the recesses 68.

A cover 72 is provided for housing 60 which includes a rectangular vertical member 73 having an upper flange 74 and lower ange 76 projecting inwardly therefrom.

Flanges 74 `and 76 are so spaced as to slidably but frictionally engage the flanges 64 and 66 to removably hold the cover 72 in place on housing 60. The iiange 74 has elongate recesses 78 formed in the end portions thereof (FIGURE 2) that are in vertical alignment with the Vrecessed portions 68 when the cover 72 is mounted on anges V64 and 66. Flange 76, as may be seen in FIG- URE 2, is shorter than flange 66 and does not extend over recesses 68 when cover 72 is mounted on housing'60` as shown' in FIGURES 4. When the cover 72 is removably mounted onfthe ilanges 64 and 66, the interior surface -of .vertical member73 is outwardly disposed fromthe exterior surf-aces of the posts E most adjacent thereto. It will vbe notedr that when the cover 72 is so mounted, the

' 'Ihe upper member 24 has an extension 24a that projects v ends thereof.

l arms P and P project upwardly through slots 78 (FIG- URE 1).

The ilange 64, as may best be seen in FIGURE 2 has an annular U-shaped slit formed therein. The portion of llange 64 within' the contines of this slit is bent downwardly to define a longitudinally extending tongue 80y that is adapted to removably engage an anchor assembly 82, the details of which are shown in FIGURES 3, 10a and 10b. An opening 84 is formed in flange 64 adjacent the tongue 88. A tongue 86 similar to tongue 81B in form but of t eavier structure, is welded or otherwise atiixed to the interior surface of flange 66 as shown in FIG- URE 3. The tongue 86 serves to removably support an anchor `assembly 82 on iiange 66, but oppositely disposed to the anchor assembly engaged by tongue 8).

Each [anchor assembly 82, as may best be seen in FIG- URES 3, 10ft and 10b, includes a mounting member 88 and anchor member 98. The mounting member 8S includes two identical parallel side walls 92 that areV connected by a cross piece 94 and an angularly disposed end piece 96. The side walls 92 have two pairs of slots 98 and 1136 formed therein, and end piece 96 has a centrally disposed bore 1612 formed therein.

The anchor member 911 includes two parallel links 104 having transversely aligned bores formed therein that are pivotally engaged by first and second cross members 166 and 168. rllhe cross members 166 and 18S have projecting end portions 18651 andV 188.4 that are slidably mounted in slots 98 and 160 respectively. A screw 11) extends through opening 182 and engages a tapped bore formed in cross member 166. Screw includes an Allen head 112. When the anchor member 96 is Idisposed inside mounting member 92, the anchor assembly is supportable' from `either flange 64 or 66 by causing the tongue 80 or 86 to slidably engage the .cross piece 94 as shown in FIGURE 3. By rotating Allen head 112 when it is in contact with end piece 96, the anchor member 90 can be caused to move towards or away from end piece 96 for reasons that will later be explained. The Allen head 112 can be rotated by a suitable tool (not shown) that is' extended through opening 84 or a similar opening (not shown) formedin flange 66.' l

Cross member 108, as can best be seen in FIGURE 10b, has a bore 4114 extending transversely therethrough. A rst helical spring 116 is .provided that has lirst and second hooks 118 and'120 respectively formed on the First hook 118 removably engages bore `114 of the anchor assembly 82 supported from flange 66-as shown in FIGURE 3. The second hook 121i engagesla loop 122 formed on one end of a cable 124 that extends upwardly to the left as illustrated' in FIGURE 3. A rigid ball 126 is mounted on the other end of cable 124. v

When the bedrails M and M' are in the upwardly extending protective position shown in FIGURE l, first portions 128 and 128 of arms P and P respectively,`

extend ldownwardly and inwardly towards one another, and are disposed at approximately a 45 angle relative to the vertical. With arms P and P in this position, second portions thereof 130 and 131i respectively, areA vertically disposedas may best lbe seen in FIGURE 3.

A tirst cam member k132 is rotatably supported on a shaft 134 that extends between web 62 and the vertical extension 70. A lug 136 is welded or otherwise axed to the shaft 134 and is also rigidly connected by Welding or other fastening means (not shown) to the extension 70, to prevent rotation'of shaft 134. The first cam member 132, as may be seen in FIGURE 3, is elongate, and the shaft .134 passes through a bore formed in the lower end portion thereof.

The upper portion of the cam member 132 is curved,

and the outer portion thereof `develops into two laterally sure contact therewith. The upper portion of the first cam member 132 slopes downwardly and develops into an inwardly disposed surface 140 that is substantially straight, as may be seen in FIGURE 3.

|Ihe second portion 130 of arm P is afixed to that side of the cam member 132 most outwardly disposed from the `headboard B by bolts 142 or other fastening means. The cam member 132 has a groove 144 formed therein (FIGURE 4) that can be engaged by the cable 124 as the arm P pivots from the upper position shown in FIG- URE 3 to the lower position shown in phantom line `in the same figure. The position of the first cam member 132 when the arm P is in the downwardly extending position is also shown in FIGURE 3 in phanton line.

When it is desired to move the bedrail M from the upwardly extending position shown in solid line in [FIG- URE l to a downwardly disposed position shown in phantom line in the same figure, the fastening member 38 is disengaged from the pin 40, and the bedrail M moved downwardly into the desired position. It will be particularly noted that while the legs 17 move through approximately 180 from the upwardly extending position to the downwardly extending position, the arm P only rotates through approximately 90. As the bedrail M moves from the up position towards the lower position, the Weight of the bedrail M ,and cabinet N supported by the arm P decreases. During this downward rotation of the bedrail M and cabinet N, the spring 104 which is initially under tension, exerts an increasing tensional force on the arm P.

Due to the elongate shape of the first cam member 132, the lever `arm between the center of rotation of the first cam member, :and that portion thereof just contacted by the cable 124 as the arm` P rotates in a counter-clockwise direction, decreases. However, this decrease in the effective lever arm is so calculated relative to the increasing force etfectedby the deforming spring 116, that a substantially constant upward force is exerted on the bedrail M and cabinet N as the rail and cabinet move from the upright to the downwardly extending position shown in phantom line in FIGURE 1. Thus it will be seen that while the tensional yforce exerted by the spring 116 varies and increases `as it deforme longitudinally due to rotation of the cam member r132 and arm P, a variable upward force is exerted on the bedrail M and cabinet N during such movement due to the shape of the cam member 132 that is just sufficient to balance the bedrail M and cabinet N in the particular position to which it has been moved.

A second helical spring 150 is provided that has a first curved end portion 1512 `which engages opening 114 in the 'anchor assembly 82 supported on fiange 60 as shown in FIGURE 3. Spring 150 includes a second curved end portion 1154 that is connected to Ia loop 156 on a cable VS which terminates in a second -ball `16). The ball 160 is removably gripped between two laterally spaced lugs 162 which are arranged in the same manner as the lugs 124 previously described. Lugs 162 are Aformed on a second cam member 166, as may 'best be seen in FIGURE 5.

The second portion 130' of arm P is affixed to the outer face of the second cam member 166 Iby bolts 168 or other suitable fastening means. A shaft 17() extends transversely between the web 62 and extension 7i), as shown in FIGURE 3. A `lug 172 is welded to the under side of the shaft 176i and rigidly affixed to the extension 70 to prevent `rotation of the shaft. Shaft 170 extends through a transverse bore 174 formed in the lower portion of the second cam member 166. The second cam member 166 has an upper portion, one side of which slopes downwardly and outwardly towards the lugs 162, and against which a portion of the cable 158 bears when the ball 160 engages the lugs 162.

The second cam` 166 has .an edge surface 176 in which a groove 178 is formed that engages the second cable 15S as the arm P pivots `from the upwardly extending position shown in solid line in FIGURE 3 to the lower position shown in phantom line in the same gure. The arm P in cooperation with the spring and second cam 166 serves the same purpose for the bedrail M as that [of the previously described portion of the counterbalance unit for bedrail M with theca-binet N supported thereon.

It will be apparent that the bedrail M does not include the cabinet -N Iand is therefore considerably lighter in Weight. Accordingly, the spring 150l used in conjunction therewith may be one that exerts substantially less force when longitudinal-ly deformed than the spring 116, yet still support the Ibedr-ail M' 4at any desired position by means of arm P. The counterbalance unit is preferably afiixed to the panel I as shown in FIGURE 1 by bolts or screws (not shown) that extend through bores 180 in web 62.

The cams 132 `and 166 lare interchangeable tand can be used on desired sides of the invention. The lugs 138 and 162 contact the .flange 166 when the bedrails M and M are in :the fully down position and Iaccordingly also function as stops.`

In FIGURE 2 it will be noted that there is a second tongue 870 .and opening 84' to theright of tongue 80 and opening 84. Also, in `FIGURE 3 it will be seen that there is a second tongue `86' to the left of tongue 86. This construction permits the anchors 82, :sp-rings 116 and 150, Vand the tarm assemblies P and P to be reversed to ailovv the bedrail M and cabinet N to be mounted on the right-hand side of the bed as lviewed in FIGURE 1, and

`the bedrail M' on the left-hand side thereof.

The operation `of the invention has been described in detail hereinabove and need not be repeated.

Although the present invention Iis -fully capable of achieving the `objects and providing the advantages hereinlbefo-re mentioned, it 4is to be understood that it is merely illustrative ofthe presently preferred embodiment thereof `and I do not mean to be limi-ted to the details of construction herein shown and described, other than as defined in the appended claims.

I claim:

1. A counterbalance unit for balancing the weight of a bedrail pivotally supported in a longitudinally extending position along `one side of a hospital fbed and extending between the headboard 'and foot-board thereof, said bedrail including at least one longitudinally extending rigid member =and being capable of occupying sa first upwardly extending protective position and in anyone of a number of second noneprotective positions in which it is disposed outwardly from said bed la greater distance than when in said first position, said unit comprising: a rigid arm having first and second tend portions; means for pivotally connecting said first end portion of said arm to said rigid member; means for removably holding said :bedrail-in said first posi-tion; -a housing; means for supporting said housing from said head-board; an elongate cam member having first and second end portions and Ia bore for-med therein that extends transversely through said cam member radjacent said second end portion; la shaft supported from said housing that extends through said transverse bore and pivotally supports said cam member; means supporting `said second end portion of said arm from la side of s-aid cam member and normal to said shaft; a cable havin-g first and second end portions; means for afiixing said first end portion `of said cable to said cani member, with a part of said cable adjacent said first end portion extending yove-r said first end portion of said cam, lwith said first end portion of said cani being upwardly disposed rwhen said bedrail is in said iirst position; and spring means in said housing connected to said second end portion of said cable for exerting ra force of constantly increasing magnitude lon said cable Ias said cam member pivots due to movement of `said bedrail trom said iirst to one of said second positions, with said cam being of such shape that as it pivots to increase said force on said spring only that 'i7 portion of said `torce -required to counterbalance said bedrail in the particular one [of said second positions in which said Ibedrail may Ibe disposed is transferred to said arm.

2. A counterbalance unit as defined in claim 1 wherein siad Imeans tor pivotally connecting said first port-ion of said arm to sai-d rigid member is an elongate cylindrical body hav-ing a cavity extending longitudinally therein from a iirs't end thereof as well as a tapped transverse bore which communicates with said cavity, and yfurther includes a-n extension of said body that projects trom a lirst end thereof, said extension Shaving threads `formed on the exterior surface thereof, which threads engage threads formed in a tapped ibore formed in an end portion lof said rigid member; a screw that engages said threads -in said tapped bore; .an elongate fastener that is insertable in said cavity -forrned in said cylindrical body; and a head mounted on the louter end yof said fastener, said fastener extending through an openingV Iiiorrned in said iirst end portion of said arm, with said Ascrew when tightened, engaging a liattened recessed por-tion tot said fastener to hold the same in said bore to nraintainjsaid arm -in pivotal engagement with said fastener Iduri-ng such ytime as said fastener is held in said cylindrical body -by said screw.

3. A counterbalance unit `as defined claim 1 which in addition includes `anchor means mounted in said housing, with said second end portion of said spring connected thereto.

4. A counterbalance unit as delined in claim 1 which in laddition includes anchor means removably mounted in said housing, with said second end portion of said spring connected thereto.

`5. A counterbal-ance unit as defined in claim 1 which in addition includes manually .adjustable anchor means mounted in said housing, with said second end porti-on of said spring connected thereto, and an opening in said housing to permit adjustment of said `anchor means for varying the tension ionsaid spring.

6. A -counterbalance unit :as `defined in claim 1 which in addition includes manually adjustable anchor means removably mounted in said housing, with said second end portion of said spring connected thereto, and said anchor means capable of occupying either of tivo positions in said housing to allow said unit to be with said bedrail when mounted on `either side of said bed.

7. A counterbalance unit as dened in claim 2 wherein said hou-sing is defined lby an elongate transverse web disposed adjacent said headboard, and further includes two flanges that extendoutwardly from `the longitudinal edges of said web, a cover that slidably, engages said anges to removably hold said cover in pla-ce there-on, which cover and iianges have recessed end portions that cooperatively define a slot through which said arm projects, with said slot permitting the pivotal movement of said armas said bedrail moves from said rst to one of said second positions.

8.y A counterbal-ance unit Ias defined in claim 2 wherein said means ttor afliX-ing said tirst end portion of said cable to said cam member comprise two laterally spaced lugs formed on the side lof said cam member `opposite .that side which will be contacted by said cable when said cam member is rotated by said arm as said bedrail -moves Ifrom said iirst to one of said second posi-tions, which unit yfurv ther includes a rigid ball mounted on said iirst end portion tot said cable that removably engages the under sides of said lugs.

References Cited in the ile of this patent UNTED STATES PATENTS 2,648,075 Cha'n `et al Allg. 11, 1953 FOREIGN PATENTS 2,196 Great Britain Peb. 15, 1886 

1. A COUNTERBALANCE UNIT FOR BALANCING THE WEIGHT OF A BEDRAIL PIVOTALLY SUPPORTED IN A LONGITUDINALLY EXTENDING POSITION ALONG ONE SIDE OF A HOSPITAL BED AND EXTENDING BETWEEN THE HEADBOARD AND FOOTBOARD THEREOF, SAID BEDRAIL INCLUDING AT LEAST ONE LONGITUDINALLY EXTENDING RIGID MEMBER AND BEING CAPABLE OF OCCUPYING A FIRST UPWARDLY EXTENDING PROTECTIVE POSITION AND IN ANY ONE OF A NUMBER OF SECOND NON-PROTECTIVE POSITIONS IN WHICH IT IS DISPOSED OUTWARDLY FROM SAID BED A GREATER DISTANCE THAN WHEN IN SAID FIRST POSITION, SAID UNIT COMPRISING: A RIGID ARM HAVING FIRST AND SECOND END PORTIONS; MEANS FOR PIVOTALLY CONNECTING SAID FIRST END PORTION OF SAID ARM TO SAID RIGID MEMBER; MEANS FOR REMOVABLY HOLDING SAID BEDRAIL IN SAID FIRST POSITION; A HOUSING; MEANS FOR SUPPORTING SAID HOUSING FROM SAID HEADBOARD; AN ELONGATE CAM MEMBER HAVING FIRST AND SECOND END PORTIONS AND A BORE FORMED THEREIN THAT EXTENDS TRANSVERSELY THROUGH SAID CAM MEMBER ADJACENT SAID SECOND END PORTION; A SHAFT SUPPORTED FROM SAID HOUSING THAT EXTENDS THROUGH SAID TRANSVERSE BORE AND PIVOTALLY SUPPORTS SAID CAM MEMBER; MEANS SUPPORTING SAID SECOND END PORTION OF SAID ARM FROM A SIDE OF SAID CAM MEMBER AND NORMAL TO SAID SHAFT; A CABLE HAVING FIRST AND SECOND END PORTIONS; MEANS FOR AFFIXING SAID FIRST END PORTION OF SAID CABLE TO SAID CAM MEMBER, WITH A PART OF SAID CABLE ADJACENT SAID FIRST END PORTION EXTENDING OVER SAID FIRST END PORTION OF SAID CAM, WITH SAID FIRST END PORTION OF SAID CAM BEING UPWARDLY DISPOSED WHEN SAID BEDRAIL IS IN SAID FIRST POSITION; AND SPRING MEANS IN SAID HOUSING CONNECTED TO SAID SECOND END PORTION OF SAID CABLE FOR EXERTING A FORCE OF CONSTANTLY INCREASING MAGNITUDE ON SAID CABLE AS SAID CAM MEMBER PIVOTS DUE TO MOVEMENT OF SAID BEDRAIL FROM SAID FIRST TO ONE OF SAID SECOND POSITIONS, WITH SAID CAM BEING OF SUCH SHAPE THAT AS IT PIVOTS TO INCREASE SAID FORCE ON SAID SPRING ONLY THAT PORTION OF SAID FORCE REQUIRED TO COUNTERBALANCE SAID BEDRAIL IN THE PARTICULAR ONE OF SAID SECOND POSITIONS IN WHICH SAID BEDRAIL MAY BE DISPOSED IS TRANSFERRED TO SAID ARM. 